Exynos: pwm: Remove dead code of function exynos5_get_pwm_clk

As we shall now be using clock_get_periph_rate function.
We find no reason for keeping code in function exynos5_get_pwm_clk.
Hence, removing it.

Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>